Generally, rise in temperature of a pneumatic tire is not favorable in view of the durability because the temperature rise accelerates over-time changes such as the changes in the properties of a material for the pneumatic tire or causes a break of a tread thereof during a high speed run. Lowering the temperature of the tire has been a significant desideratum to improve the durability especially for a run-flat tire in a case where a passenger vehicle with the run-flat tire is running while the run-flat tire is punctured (running with a 0-kPa internal pressure). For example, if a run-flat tire that has a crescent-shaped reinforcement rubber of a crescent shaped cross section taken in a tread width direction is running while being punctured, deformation in radial direction concentrates on the reinforcement rubber. This raises the temperature of this portion to a very high temperature, thereby affecting significantly the durability.
In an example of such new means for lowering tire temperature (see WO 2007/032405), the cooling effect is improved by forming turbulence generation ridges extending in the radial direction of the tire in tire side portions and by causing the turbulence generation ridges to generate or accelerate turbulence on the surface of the tire. Rubber, which tires are made of, is a material of poor heat conductivity. So, it is a known fact that the cooling effect by accelerating the turbulence generation is more effective than the cooling effect by expanding the heat-dissipating area.
In a conventional pneumatic tire having turbulence generation ridges formed therein, however, the turbulence generation ridges are formed so as to protrude from the tire side portions and are thus exposed outside. Therefore, if coming into contact with something (e.g., a curb), the tire side portions, i.e., the turbulence generation ridges may be damaged, e.g., chipped or torn away. The exposed turbulence generation ridges formed so as to protrude from the tire side portions are more likely to have defective shape or appearances for the following reason. When the exposed turbulence generation ridges are formed in the mold, bare defects are more likely to occur especially in an inner end portion on the innermost side, in the tire-radial direction, of each turbulence generation ridge, and in an outer end portion on the outermost side, in the tire-radial direction, of each turbulence generation ridge.
In addition, while a vehicle is negotiating a sharp curve and a large lateral force is acting on pneumatic tires, the outer end portions (i.e., the end portions closer to the tread portion) located on a surface of the tire come into contact with the road surface, the surface facing outward when the tire is fitted to the vehicle. Such a contact leads to an inconvenience that the tire may be damaged in the outer end portions. A countermeasure to address such inconvenience is leaving the outer-side portion, in the tire-radial direction, of each tire side portion without any turbulence generation ridges formed therein. Such a measure, however, leads to another problem that temperature cooling effect of the turbulence generation ridges is lowered.
Accordingly, an object of the invention is providing a pneumatic tire capable of reducing the possibility of defects in shape and appearances during the manufacture, capable of reducing damages on turbulence generation ridges while a vehicle is running, and capable of enhancing the cooling effect on the temperature of the tire side portions.